1. Technical Field of the Invention
The present invention relates to continuous composite coextrusion methods, apparatus for coextrusion, and compositions for preparing composites, such as continuous fiber reinforced ceramic matrix composites, using dense fibers and green matrices as well as to methods for the preparation of composites having interfaces between dense fibers and green matrices.
2. Background of the Invention
Composites are combinations of two or more materials present as separate phases and combined to form desired structures so as to take advantage of certain desirable properties of each component. The materials can be organic, inorganic, or metallic, and in various forms, including but not limited to particles, rods, fibers, plates and foams. Thus, a composite, as defined herein, although made up of other materials, can be considered to be a new material have characteristic properties that are derived from its constituents, from its processing, and from its microstructure.
Composites are made up of the continuous matrix phase in which are embedded: (1) a three-dimensional distribution of randomly oriented reinforcing elements, e.g., a particulate-filled composite; (2) a two-dimensional distribution of randomly oriented elements, e.g., a chopped fiber mat; (3) an ordered two-dimensional structure of high symmetry in the plane of the structure, e.g., an impregnated cloth structure; or (4) a highly-aligned array of parallel fibers randomly distributed normal to the fiber directions, e.g., a filament-wound structure, or a prepreg sheet consisting of parallel rows of fibers impregnated with a matrix.
Monolithic ceramic materials are known to exhibit certain desirable properties, including high strength and high stiffness at elevated temperatures, resistance to chemical and environmental attack, and low density. However, monolithic ceramics have one property that limits their use in stressed environments, namely their low fracture toughness. While significant advances have been made to improve the fracture toughness of monolithic ceramics, mostly through the additions of whisker and particulate reinforcements or through careful control of the microstructural morphology, they still remain extremely damage intolerant. More specifically, they are susceptible to thermal shock and will fail catastrophically when placed in severe stress applications. Even a small processing flaw or crack that develops in a stressed ceramic cannot redistribute or shed its load on a local scale. Under high stress or even mild fatigue, the crack will propagate rapidly resulting in catastrophic failure of the part in which it resides. It is this inherently brittle characteristic which can be even more pronounced at elevated temperatures, that has not allowed monolithic ceramics to be utilized in any safety-critical designs.
Research and development for these high temperature and high stress applications have focused on the development of continuous fiber reinforced ceramic matrix composites, hereafter referred to as CFCCs. The use of fiber reinforcements in the processing of ceramic and metal matrix composites is known in the prior art, and has essentially provided the fracture toughness necessary for ceramic materials to be developed for high stress, high temperature applications. See J. J. Brennan and K. M. Prewo, xe2x80x9cHigh Strength Silicon Carbide Fiber Reinforced Glass-Matrix Composites,xe2x80x9d J. Mater. Sci., 15 463-68 (1980); J. J. Brennan and K. M. Prewo, xe2x80x9cSilicon Carbide Fiber Reinforced Glass-Ceramic Matrix Composites Exhibiting High Strength Toughness,xe2x80x9d i J. Mater. Sci., 17 2371-83 (1982); P. Lamicq, G. A. Gernhart, M. M. Danchier, and J. G. Mace, xe2x80x9cSiC/SiC Composite Ceramics,xe2x80x9d Am. Ceram. Soc. Bull., 65 [2] 336-38 (1986); T. I. Mah, M. G. Mendiratta, A. P. Katz, and K. S. Mazdiyasni, xe2x80x9cRecent Developments in Fiber-Reinforced High Temperature Ceramic Composites,xe2x80x9d Am. Ceram. Soc. Bull., 66 [2] 304-08 (1987).; K. M. Prewo, xe2x80x9cFiber-Reinforced Ceramics: New Opportunities for Composite Materials,xe2x80x9d Am. Ceram. Soc. Bull., 68 [2] 395-400 (1989); H. Kodama, H. Sakamoto, and T. Miyoshi, xe2x80x9cSilicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites,xe2x80x9d J. Am. Ceram. Soc., 72 [4] 551-58 (1989); and J. R. Strife, J. J. Brennan, and K. M. Prewo, xe2x80x9cStatus of Continuous Fiber-Reinforced Ceramic Matrix Composite Processing Technology,xe2x80x9d Ceram. Eng. Sci. Proc., 11 [7-8] 871-919 (1990).
Under high stress conditions, the fibers are strong enough to bridge the cracks which form in the ceramic matrix allowing the fibers to ultimately carry the load, and catastrophic failure can be avoided. This type of behavior has led to a resurgence of CFCCs as potential materials for gas turbine components, such as combustors, first-stage vanes, and exhaust flaps. See D. R. Dryell and C. W. Freeman, xe2x80x9cTrends in Design in Turbines for Aero Engines,xe2x80x9d pp. 38-45 in Materials Development in Turbo-Machinery Design; 2nd Parsons International Turbine Conference, Edited by D. M. R. Taplin, J. F. Knott, and M. H. Lewis, The Institute of Metals, Parsons Press, Trinity College, Dublin, Ireland, 1989. CFCCs have also been given serious consideration for heat exchangers, rocket nozzles, and the leading edges of next-generation aircraft and reentry vehicles. See M. A. Karnitz, D. F. Craig, and S. L. Richlin, xe2x80x9cContinuous Fiber Ceramic Composite Program,xe2x80x9d Am. Ceram. Soc. Bull., 70 [3] 430-35 (1991), and Flight Vehicle Materials, Structures and Dynamicsxe2x80x94Assessment and Future Directions, Vol. 3, edited by S. R. Levine, American Society of Mechanical Engineers, New York, 1992. In addition, CFCCs with a high level of open porosity are currently being utilized as filters for hot-gas cleanup in electrical power generation systems, metal refining, chemical processing, and diesel exhaust applications. See L. R. White, T. L. Tompkins, K. C. Hsieh, and D. D. Johnson, xe2x80x9cCeramic Filters for Hot Gas Cleanup,xe2x80x9d J. Eng. for Gas Turbines and Power, Vol. 115, 665-69 (1993).
CFCCs are currently fabricated by a number of techniques. The simplest and most common method for their fabricating has been the slurry infiltration technique whereby a fiber or fiber tow is passed through a slurry containing the matrix powder; the coated fiber is then filament wound to create a xe2x80x9cprepregxe2x80x9d; the prepreg is removed, cut, oriented, and laminated into a component shape; and the part undergoes binder pyrolysis and a subsequent firing cycle to densify the matrix. See J. J. Brennan and K. M. Prewo, xe2x80x9cHigh Strength Silicon Carbide Fibre Reinforced Glass-Matrix Composites,xe2x80x9d J. Mater. Sci., 15 463-68 (1980); D. C. Phillips, xe2x80x9cFiber Reinforced Ceramics,xe2x80x9d Chapter 7 in Fabrication of Composites, edited by A. Kelly and S. T. Mileiko, North-Holland Publishing Company, Amsterdam, The Netherlands, 1983; and K. M. Prewo and J. J. Brennan, xe2x80x9cSilicon Carbide Yarn Reinforced Glass Matrix Composites,xe2x80x9d J. Mater. Sci., 17 1201-06 (1982).
Other techniques for fabricating CFCCs also typically involve an infiltration process in order to incorporate matrix material within and around the fiber architecture, e.g. a fiber tow, a preformed fiber mat, a stack of a plurality of fiber mats, or other two dimensional (2D) or three dimensional (3D) preformed fiber architecture. These techniques include the infiltration of sol-gels. See J. J. Lannutti and D. E. Clark, xe2x80x9cLong Fiber Reinforced Sol-Gel Derived Alumina Compositesxe2x80x9d, pp. 375-81 in Better Ceramics Through Chemistry, Material Research Society Symposium Proceedings, Vol. 32, North-Holland, New York, 1984; E. Fitzer and R. Gadow, xe2x80x9cFiber Reinforced Composites Via the Sol-Gel Routexe2x80x9d, pp. 571-608 in Tailoring Multiphase and Composite Ceramics, Materials Science Research Symposium Proceedings, Vol. 20, edited by R. E. Tressler et al., Plenum Press, New York, 1986. Other techniques include polymeric precursors which are converted to the desired ceramic matrix material through a post-processing heat treatment. See J. Jamet, J. R. Spann, R. W. Rice, D. Lewis, and W. S. Coblenz, xe2x80x9cCeramic-Fiber Composite Processing via Polymer-Filler Matrices,xe2x80x9d Ceram. Eng. Sci. Proc., 5 [7-8] 677-94 (1984); and K. Sato, T. Suzuki, O. Funayama, T. Isoda, xe2x80x9cPreparation of Carbon Fiber Reinforced Composite by Impregnation with Perhydropolysilazane Followed by Pressureless Firing,xe2x80x9d Ceram. Eng. Sci. Proc., 13 [9-10] 614-21 (1992).
Other research and development has involved molten metals that are later nitrided or oxidized. See M. S. Newkirk, A. W. Urquhart, H. R. Zwicker, and E. Breval, xe2x80x9cFormation of Lanxide Ceramic Composite Materials,xe2x80x9d J. Mater. Res., 1 81-89 (1986); and M. K. Aghajanian, M. A. Rocazella, J. T. Burke, and S. D. Keck, xe2x80x9cThe Fabrication of Metal Matrix Composites by a Pressureless Infiltration Technique,xe2x80x9d J. Mater. Sci., 26 447-54 (1991). Other research and development has involved molten materials that are later carbided to form a ceramic matrix. See R. L. Mehan, W. B. Hillig, and C. R. Morelock, xe2x80x9cSi/SiC Ceramic Composites: Properties and Applications,xe2x80x9d Ceram. Eng. Sci. Proc., 1 405 (1980). Still other research and development has involved molten silicates that cool to form a glass or glass-ceramic matrix (see M. K. Brun, W. B. Hillig, and H. C. McGuigan, xe2x80x9cHigh Temperature Mechanical Properties of a Continuous Fiber-Reinforced Composite Made by Melt Infiltration,xe2x80x9d Ceram. Eng. Sci. Proc., 10 [7-8] 611-21 (1989)), and chemical vapors which decompose and condense to form the ceramic matrix (See A. J. Caputo and W. J. Lackey, xe2x80x9cFabrication of Fiber-Reinforced Ceramic Composites by Chemical Vapor Infiltration,xe2x80x9d Ceram. Eng. Sci. Proc., 5 [7-8] 654-67 (1984); and A. J. Caputo, W. J. Lackey, and D. P. Stinton, xe2x80x9cDevelopment of a New, Faster, Process for the Fabrication of Ceramic Fiber-Reinforced Ceramic Composites by Chemical Vapor Infiltration,xe2x80x9d Ceram. Eng. Sci. Proc., 6 [7-8] 694-706 (1985).
Two U.S. patents have issued which involve a method for the fabrication of a fiber reinforced composite by combining an inorganic reinforcing fiber with dispersions of powdered ceramic matrix in organic vehicles, such as thermoplastics. The first patent, U.S. Pat. No. 5,024,978, discloses a method for making an organic thermoplastic vehicle containing ceramic powder that can form the matrix of a fiber reinforced composite. This patent also discloses that the ceramic powder/thermoplastic mixtures can be heated to above the melt transition temperature of the thermoplastic and then applied as a heated melt to an inorganic fiber. This patent further discloses that the process may be used to make composite ceramic articles. The second patent, U.S. Pat. No. 5,250,243, discloses a method for applying a dispersion of ceramic powder in a wax-containing thermoplastic vehicle to an inorganic fiber reinforcement material to form a prepreg material such as a prepreg tow. This patent further discloses that the prepreg tow may be subjected to a binder pyrolysis step to partially remove the wax binder vehicle prior to consolidation of the prepreg tow into the preform of a composite ceramic article.
To summarize, the continuous fiber reinforced ceramic composites (xe2x80x9cCFCCsxe2x80x9d) prior to the present invention have traditionally been fabricated using methods and apparatuses to infiltrate the matrix or matrix-forming material around a preformed architecture of dense fibers or fiber tows or by passing the fibers through a powder/melt slurry. While these methods and apparatuses provide a fiber reinforced composite structure, there is no control over the thickness of the matrix forming vehicle, and rarely will the matrix uniformly surround the fibers. In such methods, the fibers often contact each other which is detrimental to the mechanical behavior of such composites. In addition, these infiltration processes are quite slow, sometimes requiring weeks or months to fabricate components, and are severely limited in the matrix/fiber combinations that can be produced.
Thus, there exists a need for more efficient methods and apparatuses for applying the matrix to the fiber reinforcement. There exists a further need for methods and apparatuses that are versatile enough to allow almost limitless combinations of matrix and fiber reinforcement.
It is therefore an object of the present invention to provide methods and apparatuses for efficient fabrication of ceramic composites that exhibit non-catastrophic behavior when used as a fiber reinforcement for a green ceramic matrix.
Another object of the present invention is to provide relatively efficient methods and apparatuses for applying the green matrix material to the fiber reinforcement such that it completely surrounds the fiber reinforcement prior to composite layup.
A further object of the present invention is to provide relatively efficient methods and apparatuses for preparing and applying the green matrix material to the fiber reinforcement, regardless of the composition from which the matrix is prepared or the composition of the fiber reinforcement.
Yet another object of the present invention is to provide relatively efficient methods and apparatuses for preparing both a green ceramic matrix and a green matrix/fiber interfacial layer that can be applied to the fiber reinforcement regardless of the composition of the matrix, interface, or fiber reinforcement.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent to those of skill in the art from the description of the invention provided herein.
The present invention comprises novel continuous composite coextrusion methods and apparatus for fabricating fiber reinforced composite materials. Specifically, the present invention comprises novel methods and apparatus to fabricate composite materials via an economical, versatile, and controlled continuous composite coextrusion processes. In a particular preferred embodiment of the present invention, a dense fiber or dense fiber tow (bundles of fibers) is introduced during melt extrusion of a ceramic (or metal)/binder feed-rod. The result of this coextrusion process is a coextruded xe2x80x9cgreenxe2x80x9d filament containing an in-situ dense fiber or tow of fibers.
More specifically, the present invention relates to processes for the fabrication of a fiber reinforced composite, i.e., a composite which is comprised of a matrix of a material, such as a ceramic or metallic material, and having fibers of a ceramic material dispersed within the matrix as a reinforcement. A preferred method of the present invention comprises: (a) forming a material-laden composition comprising a thermoplastic polymer and at least about 40 volume % of a ceramic or metallic particulate in a manner such that the composition has a substantially cylindrical geometry and thus can be used as a substantially cylindrical feed rod; (b) forming a hole down the symmetrical axis of the feed rod; (c) inserting the start of a continuous spool of ceramic fiber, metal fiber or carbon fiber through the hole in the feed rod; (d) extruding the feed rod and fiber reinforcement simultaneously to form a continuous filament consisting of a xe2x80x9cgreenxe2x80x9d matrix material completely surrounding a dense fiber reinforcement and said filament having an average diameter that is less than the average diameter of the feed rod; and (e) arranging the continuous filament into a desired architecture to provide a green fiber reinforced composite. The green matrix may be subsequently fired, i.e., heated, to provide a fiber reinforced composite with non-brittle failure characteristics.
The present invention also provides a process for the fabrication of a fiber reinforced composite having an interlayer, i.e., a composite that is comprised of a matrix of material, such as a ceramic or metallic material, having fibers of a ceramic material dispersed within the matrix as a reinforcement, and having an interlayer that is between the matrix and fiber reinforcement. This method is the same as that described in the preceding paragraph, but further comprises forming a feed rod that contains two dissimilar particulate-laden compositions wherein during the extrusion process the second particulate-laden composition forms a green interlayer between the fiber reinforcement and the green matrix in a continuous filament. This filament can be arranged as described in the previous paragraph and both the green interlayer and the green matrix may be subsequently fired to provide a fiber reinforced composite having substantially improved non-brittle failure characteristics compared to a fiber reinforced composite in the absence of an interlayer.
The present invention further provides methods for the fabrication of continuous filaments used in preparing fiber reinforced composites wherein the architecture of the filaments can be readily controlled.
Yet another aspect of the present invention is the ability to take the continuous filaments and form a shaped green-body. Typically, the extruded filament is molded by pressing into an appropriate mold at temperature of at least about 80xc2x0 C. The molding operation joins the fiber reinforced green filaments together, creating a solid, shaped green body. Any shape that can be compression molded or otherwise formed by plastic deformation can be obtained with extruded filament. The green body so molded has the desired texture created by the arrangement of the extruded filaments. For example, a uniaxially aligned fiber reinforced composite can be obtained by a uniaxial lay-up of the extruded filaments prior to molding, or a woven architecture can be obtained by molding a shape from previously woven extruded filaments. The extruded filament product permits a wide variety of composite architectures to be fabricated in a molded green body.
In a preferred method of the present invention, a co-axial filament is produced with a fiber tow surrounded by a xe2x80x9cgreenxe2x80x9d ceramic. In a further preferred embodiment of the present invention, the process has been demonstrated utilizing carbon fiber tows in a hafnium carbide (xe2x80x9cHfCxe2x80x9d) matrix and the resulting product can be used in extreme high temperature environments. The fiber imparts the necessary thermal shock resistance and toughness that HfC lacks as a monolithic ceramic.
The processing techniques of the invention readily allows for control of the fiber volume fraction and changes to the matrix composition. This technology is readily applicable to other matrix/fiber combinations and will significantly enhance manufacturing capability for low cost, high-performance and high temperature ceramic composites.